Method and composition for removal of soot and deposits from heat exchange surfaces of combustion units

ABSTRACT

There is disclosed a method for removal of soot and deposits from heat exchange surfaces of combustion units. The method provides that the combustion unit is charged with a substance capable of forming a catalytically inactive compound with iron and vanadium oxides while combustion conditions are maintained. Operation of the combustion unit is continued at its normal operating temperature for a period of time sufficient to cause the deposits to crack and/or to loosen from the surface to be cleaned. There is further disclosed a suitable composition for carrying out the aforedescribed method.

United States Patent Mallen June 3, 1975 [54] METHOD AND COMPOSITION FOR 2,844,112 7/1958 Muller 110/1 REMOVAL OF SOOT AND DEPOSITS FROM g t 1 6 8C HEAT EXCHANGE SURFACES OF 3.234898 2/1966 M61666 110/1 COMBUSTION UNITS 2/1967 Lewis 6161 110/1 Attorney, Agent, or Firm-Hane, Baxley & Spiecens ABSTRACT There is disclosed a method for removal of soot and deposits from heat exchange surfaces of combustion units. The method provides that the combustion unit is charged with a substance capable of forming a catalytically inactive compound with iron and vanadium oxides while combustion conditions are maintained. Operation of the combustion unit is continued at its normal operating temperature for a period of time sufficient to cause the deposits to crack and/or to loosen There is further disclosed a suitable composition for 5 Claims, 1 Drawing Figure [75] Inventor: Walter Tony Mallen, Orebro,

Sweden Primary Examiner-Kenneth W. Sprague [73] Assignee: Nitro Nobel AB, Gyttorp, Sweden [22] Filed: Mar. 20, 1973 [57] [21] Appl. No.: 342,931

[30] Foreign Application Priority Data Mar. 25, 1972 United Kingdom 14105/72 [52] US. Cl. 110/1 J; 44/4; 44/5; 110/1 K [51] Int. Cl C101 9/02 [58] Field of Search 165/1;110/1 K, 1.1;44/4, 44/5 from the surface to be cleaned.

[56] References Cited UNITED STATES PATENTS carrying out the aforedescribed method.

1,609,059 ll/l926 Claus 44/5 2,014,686 9/1935 Lubovitch et a1 44/4 2,412,809 12/1946 Harlow 165/1 2,673,145 3/1954 Chandler 110/1 X so z o 1 z 3 4- 5 s 7 s 9 1o PATENTEDJUHB METHOD AND COMPOSITION FOR REMOVAL OF SOOT AND DEPOSITS FROM HEAT EXCHANGE SURFACES OF COMBUSTION UNITS The present invention relates to a method and a composition for the removal of soot and deposits from heat exchange surfaces of combustion units. The term combustion units is intended to include direct fired combustion units such as steam boilers and exhaust gas units.

BACKGROUND In the operation of combustion units, loose, unconsolidated carbon or soot is deposited on heat transfer surfaces as well as flues and chimneys. Such loose or unconsolidated soot is generally defined as A Finely divided powder produced during combustion of coal or wood and consisting essentially of carbon, but containing some tar and ash, etc. See the Condensed Chemical Dictionary, sixth Edition 1961, Reinhold. It is known in the prior art to utilize certain compositions to remove such loose soot from chimneys and the like. Such a removal can be made without difficulties.

Much more difficult to remove from such heat exchange surfaces and most detrimental to the operation and efficiency of the combustion unit are the hard deposits normally formed on said surfaces. These deposits comprise essentially inorganic compounds present in the fuel as well as minor amounts of carbon and carboniferous compounds, sulfur compounds, such as ferrous sulfide, sulfide trioxide, etc. Sulfur trioxide, sulfuric acid and alkalimental salts thereof are considered to function in such a way as to cement together the componets of the ash to a very hard surface coating on the exchange surfaces.

It has been found that the content of sulfur in the fuel, such as an oil, represents a very important factor in the formation of hard deposits on the heat exchange surfaces. In the combustion of the fuel, the sulfur is primarly converted into sulfur dioxide which, however, appears relatively harmless from the viewpoint of forming hard coatings. However, there are certain ingredients in the ash of the fuel that present a catalytical effect on the sulfur dioxide to convert the latter into sulfur trioxide, said compound functioning as mentioned above as a cement or binder for the ash deposited on the heat exchange surfaces. It has been found that the compounds that are most active as catalysts for the conversion of sulfur dioxide into sulfur trioxide are oxides of iron and vanadium such as ferric oxide and vanadium pentoxide. The catalytical formation of sulfur trioxide depends on many factors, such as the temperature in the combustion zone, the excess of oxygen used for the combustion and the amount of catalysts present in the combustion. Many of these factors are not possible to influence so that there is always necessarily formed a certain quantity of sulfur trioxide which may vary in the range of from to 7 percent of the sulfur dioxide content.

THE INVENTION The present invention is based on preventing the catalytically active compounds, especially oxides of iron and vanadium such as ferric oxide and vanadium oxide, from exerting their catalytical effect on the sulfur trioxide formation process. To that effect, the present agent consists of a mixture comprising at least one compound which is capable, under the combustion conditions, of forming a catalytically inactive compound with especially the oxides of iron and vanadium. It has been found, according to the invention, that, amongst others, most boron compounds inorganic and organic fulfill the requirement of reacting with the mentioned catalytically active substances under the given conditions to convert them into borates which are relatively inactive compounds.

According to the invention the content of the compound capable of suppressing the SO; formation of the composition may be in the range of from 0.5 to 15 percent by wieght of the whole mixture.

According to the invention, the effectiveness of the agent may be increased considerably if it contains one or more substances capable of reacting with sulfur trioxide to convert it into compounds not functioning as cements in the ash coating or one or more substances capable of forming substances acting in this manner. Of special interest is an agent consisting of a mixture comprising in addition to the substance capable of forming an inactive compound such as borate with ferric oxide and/or vanadium oxide, also an alkali metal nitrate and one or more oxidizable substances, the mixture presenting an oxygen over-balance in the range of from 5 to 35 percent by weight of the mixture. It is known that such an oxygen over-balance mixture. of alkali metal nitrate and one or more oxidizable compounds when introduced into a combustion unit is vaporized to form a smoke or fog under the combustion conditions of the unit, said smoke or fog containing molten particles of alkali metal nitrate and/or nitrite which latter will have a cleaning effect on the heat-transfer tubes. The combination of a component capable of suppressing the S0 formation in the combustion process with a mixture of a nitrate and one or more oxygen-consuming substances results in a very effective cleaning of tubes coated with deposits.

In calculating the excess of oxygen in a given composition, it is assumed that complete oxidation to sulfur dioxide and/or carbon diox-idetakes place and that the nitrate is reduced to its oxide (K 0, for example) and elementary nitrogen.

The present composition can be used for cleaning the surfaces of boilers, furnaces and the like which are fired by oil, coal, wood or other fuels, said fuels containing sulfur dioxide in a combustion process.

The present invention also encompasses a method for removing deposits from heat exchange surfaces of combustion units, comprising charging to said combustion unit a substance capable of reacting with oxides of iron and/or vanadium, such as ferric oxide and vanadium pentoxide, to form catalytically inactive compounds therewith, under combustion conditions and continuing operation of said combustion unit at its normal operating temperature for a sufficient period of time to cause the deposits to crack and/or loosen from the said surfaces. Such a treatment usually requires a time period amounting to at least one day. In many cases, the treatment periods amounting to several days or one or more weeks are required. The cracked and loosened fused ash can be removed by brushing the surface or by blowing the surface with a gas under pressure.

Example In order to verify the effect obtained according to the invention the following composition was made up:

potassium nitrate 93 .09? CHARCOAL 7.0% oxygen overbalance 18.0%

The composition is in the following referred to as SP.

Sample Blank volume of flue gases content of S mg concentration of S0 mg per cubic feet The following mixtures were made up:

1. SP l percent of borax (chemical formula Na B- 2. SP 3 percent of borax 3. SP 5 percent of borax 4. SP percent of borax.

The mixtures were tested on the ability of suppressing the sulfur trioxide formation in the combustion of sulfur containing fuels (fuel oil No. 4) the contents of sulfur being approximately 2.5 percent. The experiments were carried out in the main combustion unit of the Nitro Nobel AB in Gyttorp, Kingdom of Sweden.

To this effect, the sulfur trioxide of the flue gases from the combustion unit were collected in a bubbler containing an aqueous 80 percent solution of isopropylalcohol by suction produced by a water suction pump. The sulfur trioxide was estimated by the phototurbidimetric method, the volume of the flue gases sampled was measured by a M; cubic feet laboratory wet test gas meter.

One blank test was carried out without charging neither SP nor sodium borate to the furnace. One test was carried out with only SP, according to the following table. The content of S0 in the flue gases formed in the combustion of the sulfur-containing fuel oil without addition of SP or sodium borate has been assumed to be 100 percent.

The following test results were obtained:

SP SP+l7rB SP+37rB SP+57'B SP+10%B On the enclosed drawing, the percent of S0 is the flue gases compared with blank has been plotted against the percent of B added to SP.

What is claimed is:

1. A method for the removal of soot and deposits from heat exchange surfaces of a combustion unit comprising charging to said combustion unit while operating a mixture of more than fifty percent of a nitrate and a minor percent of borax, said mixture being capable of forming a catalytically inactive compound with iron and vanadium oxides under combustion conditions, and continuing the operation of said combustion unit at its normal operating temperature for a sufficient period of time to cause the deposits to crack and/or loosen from the said surfaces.

2. A method as claimed in claim 1, wherein the cracked and loosened deposits are removed by brushing the surface.

3. A method as claimed in claim 1, wherein the loosened and cracked deposits are removed by blowing the surface with a gas under pressure.

4. A method as claimed in claim 1 wherein said mixture comprises at least an alkali metal nitrate and one or more oxidizable substances, the oxygen overbalance of the mixture being in the range of from 5 to 35 percent by weight of the whole mixture.

5. A method as claimed in claim 4 wherein said oxidizable substance is charcoal. 

1. A METHOD FOR THE REMOVAL OF SOOT AND DEPOSITS FROM HEAT EXCHANGE SURFACES OF A COMBUSTION UNIT COMPRISING CHARGING TO SAID COMBUSTION UNIT WHILE OPERATING A MIXTURE OF MORE THAN FIFTY PERCENT OF A NITRATE AND A MINOR PERCENT OF BORAX, SAID MIXTURE BEING CAPABLE OF FORMING A CATALYTICALLY INACTIVE COMPOUND WITH IRON AND VANADIUM OXIDES UNDER COMBUSTION CONDITIONS, AND CONTINUING THE OPERATION OF SAID COMBUSTION UNIT AT ITS NORMAL OPERATING TEMPERATURE FOR A SUFFICIENT PERIOD OF TIME TO CAUSE THE DEPOSITS TO CRACK AND/OR LOOSEN FROM THE SAID SURFACES.
 1. A method for the removal of soot and deposits from heat exchange surfaces of a combustion unit comprising charging to said combustion unit while operating a mixture of more than fifty percent of a nitrate and a minor percent of borax, said mixture being capable of forming a catalytically inactive compound with iron and vanadium oxides under combustion conditions, and continuing the operation of said combustion unit at its normal operating temperature for a sufficient period of time to cause the deposits to crack and/or loosen from the said surfaces.
 2. A method as claimed in claim 1, wherein the cracked and loosened deposits are removed by brushing the surface.
 3. A method as claimed in claim 1, wherein the loosened and cracked deposits are removed by blowing the surface with a gas under pressure.
 4. A method as claimed in claim 1 wherein said mixture comprises at least an alkali metal nitrate and one or more oxidizable substances, the oxygen over-balance of the mixture being in the range of from 5 to 35 percent by weight of the whole mixture. 